text:
Can flaps be used to perform rotation and flare?
TL; DR using flaps to directly control sink/climb rate near ground = simpler and faster response = easy to fly airplane?
Rotating is the pitch up during takeoff that causes the aircraft to leave the runway.
Flare is the pitch up just before touchdown that causes the sink rate to drop.
In both maneuvers, a downforce is applied to the tail, to re orient the aircraft to increase the angle of attack, so as to increase lift. The objective is this lift, rather than a change in orientation.
Hence, would it not make sense to use a method to directly alter lift, if available? Specifically, would it not make sense to increase the flap settings just for the moment these maneuvers are done? Takeoff is done at a non-maximum flap setting. When the airspeed nears the required value for liftoff, flaps could be lowered to full. Final approach can be done with takeoff flaps, and just above the runway, instead of pulling back, the flaps could be lowered.
I know the flaps are slow to deploy, but the moment of the fuselage makes pitching slow as well, doesn't it? Even if the slow movement of the flaps erodes any advantage in terms of response time, the decreased angle of attack at the leading edge would be favorable in terms of stall characteristics, wouldn't it? And the requirements to touch the main gear first on landing could be satisfied with even a relatively small pitch angle. And tail strikes could be avoided.

You'll find during landing that the aircraft will balloon, not pitch up if you try to dump full flaps in ground effect.

Is there a problem you are trying to solve?  Because I don't think tail strikes alone are a good reason to throw out proven techniques refined and largely perfected over the last 100 plus years.  And I think the objective is as much about orienting the aircraft as gaining lift.  Particularly on rotation where you point the aircraft up where you want to go...

It's a bit like looking at a steering wheel in a car and saying "you know what, let's replace that with differential braking to control direction instead". Only your proposal is quite a bit more complicated and risky, for minimal benefit.

While this is an interesting idea, there are some key issues with it. As a GA pilot, here are my issues with that concept.

Changing flap settings causes signifigant differences in aircraft handling. If I was, say, on short final, and I changed my aircraft's configuration - it would be difficult to maintain a stable approach profile to the runway. If I were to deploy my flaps just above the runway, the aircraft would lose its "stable approach" and I would not have as much control authority over where, when, and how my aircraft touches down. There is a reason GA and Commercial pilots alike are taught to maintain a stable approach and have the aircraft fully configured well before landing.

I would never adjust flaps on take off or climb out (until at a reasonable altitude). The reason is simple. First of all, aircraft have operating limitations. You would not take off with full flaps because they generate too much drag, and hence limit aircraft performance. They are great for landing because of that same drag. However, If I were to take off with (theoretically) full flaps in a Cherokee or Cessna, it would not be safe to retract them until sufficient airspeed and altitude has been attained. Raising flaps at low airspeeds causes a noteworthy forward pitching motion and a temporary arrest of the climb rate. This is dangerous when in a low-airspeed, low-alitude situation and is warned against by CFIs.

Doesn't pitching also mess with the approach profile?

If you are referring to adjusting pitch for the flare and roundout, no - not when correctly managing airspeed. The final roundout and flare work in the first place because of the airspeed management and gentle adjustments of pitch throughout the approach to stay on the glideslope/path.

On gliders you will change flaps during take-off, for example in an aerotow. You start with positive flaps to lift off early and gradually switch to negative flaps to adjust for the increasing speed until the towplane has taken off, too.

On commercial airliners at least, flaps respond more slowly than elevators. I would worry about trying to do anything that requires certain timing with a control that doesn’t respond immediately. (This may be encapsulated in point 1)

@PeterKämpf that's true, and gliders are a totally different ball game. Good point though!

@ToddWilcox that's true - and also more true in GA than one might think. In a C172/182, flaps are most often manual and take a few seconds to deploy. In a PA-28, the flaps are manually operated and pretty much instantaneous. In both cases, even just one notch of flaps may have a signifigant impact on the aircraft's pitch, descent profile, and airspeed. Great point!

@Peter Kampf, but the glider won't start the roll at zero flaps, and put them down at rotation speed to get airborne in lieu of pitching up as the OP is suggesting.  The example you described is just normal cleaning up while accelerating once airborne.

@MichaelHall There is more that that. You start the ground roll with negative flaps to be able to control roll attitude at the lowest possible speed and change to positive flaps when fast enough for lift-off. Then you do your cleaning up. And no, flaps are not used for pitch control but to adjust the wing to the current needs.

Got it, thanks.  I confess I've only been in a glider once or twice about 30 years ago...

Yes, but.
Flaps will change the pitch equilibrium to  nose-up when moved up, so when you want to increase your lift coefficient, you need to reduce the maximum lift coefficient to make the airplane pitch up. This makes flaps much less suitable for take-off and landing than the elevator, but you can control airplanes with flaps only under some conditions. You can lower flaps momentarily to increase lift, but this will make the airplane pitch down. Doing this shortly before touchdown will in effect result in a hard landing: The airplane will initially climb with the flap change and then gradually lower its nose and consequently drop to the runway like a brick.  This is the same effect that makes landing a flying wing tricky.
There were several fatal crashes of the ASW-20 glider when the elevator link had been forgotten to be connected. This can happen after the airplane has been disassembled for road transport or maintenance and, upon reassembly, the pushrod was not re-connected to the elevator control horn.
Gerhard Waibel became so upset that he personally drove to a glider club where that happened and demonstrated how to fly an ASW-20 with a disconnected elevator.
The flaps have less control authority than the elevator, so you need to think ahead a bit. Also, they will reduce the maximum lift coefficient when commanding a nose-up pitch, so the landing will be at a higher speed. But since the flaps of gliders are controlled manually, they will be sufficient for pitch control, including landing.
This might be different on airplanes with hydraulic or electric flaps where the speed of flap angle change is much slower than the short period mode, so that pitch changes will result in PIOs or even a runaway pitch change which cannot be counteracted fast enough before control is lost.
The shortest take-off technique for the Me-262 used flaps up during the takeoff run to minimize drag until the airplane approached lift off speed. Then the flaps were lowered in order to lift off at the lowest possible speed. This is generally not recommended - when the flaps malfunction, the take-off run will become a lot longer.

Can you use control surfaces to control an aircraft? Yes.
Are flaps controls a way to control the control surfaces? Yes.
Can you use them to control an aircraft? Yes.
Could you land this way? Possibly.
Is this something you should try? No.

Flaps are a slow, clumsy and coarse method to modify your control surfaces and there are all sorts of attendant problems with this method, especially in exactly the conditions likely to dominate while landing. Last resort. Better than no control at all, I suppose. As a thought experiment it's interesting.
If you want to try it out, use a simulator.

Or try it out at altitude. I make sure that I know how control changes like sudden flap changes affect flight with every new type I fly. That knowledge might come handy in future situations.

You have a point. The question doesn't actually say land, it only mentions flare and rotation. Which you can certainly do at altitude. You could even slow to landing speed and put the gear down. Personally I'd try it in a sim first and then at altitude.

Even if we disregard the problems current flap systems have if they were to be used as flare controls (speed of operation, lag, discrete positions), and assumed they could be controlled in the same manner as flaps, there are still (at least) two major downsides:
Flaring with pitch control (the elevator) is superior in simplicity. Just pull on the stick or yoke, and that's it. If flare was initiated with flaps, you'd have one extra contorl to operate, and being a creature with two hands, you run out of interfaces with the plane. Remember, during flare you should have hands on throttle and stick.
If that problem was somehow mitigated, there's the problem of control uniformity across different types of planes. Different planes react pretty much in the same way to pitch inputs, but flaps, not necessarily.
Both of the aforemenrioned could be overcome by help of automation that would "level" out the differences and handle some of the control, but better not let Boeing design that system.
All in all KISS is a good principle. We have a perfectly functional and redundant way of managing flare.

The question is more complex than one might think, at first blush.  Can lift be lessened, in order to land, or assure a landing?  Of course.  It's the reason that spoilers deploy after landing, either manually or with auto ground spoilers, after main gear touchdown; to kill lift, put weight on the wheels, improve braking effectiveness, and to increase ground control by virtue of the gear, as aerodynamic speed decreases.
There are practical issues associated with retracting flaps during landing.  If one increases flap setting, one increases both lift and drag. If one reduces flap setting, one reduces lift and drag, and also increases stall speed.  Because approach speed is a percentage value above stall speed, this also changes the relevant approach speed.  Because the percentage margin above stall speed, approach speed, also directly relates to stall margin, it impacts descent path, angle of attack and other factors, including power required.
When flaps are lowered, the slipstream angle behind the flaps is also altered.  This alters the downwash on the horizontal stabilizer.  A change in downwash on the horizontal stabilizer affects pitch, pitch trim, and controlability for a given power setting, weight, blance, and speed.  In certain extreme cases (carrying a load of tailplane icing, for example), a change in flap setting can cause a control departure or significant pitching moment (eg, nosedive).  Outside of such extreme conditions, a change in flap setting will necessitate a speed change, which will also require a pitch change and in most cases, a power change.  Simply retracting or extending flaps during takeoff or landing is not an isolated event: it invokes every other element of the approach control as well, including requiring a pitch change.
Another aspect of changing flap setting on the runway or while approaching the runway is the possibility of moving the wrong control.  I know a crew that retracted gear upon landing, thinking it was flap, and subsequently had a gear retraction on the go...the aircraft settled back, ended up overturning, and burned.  Crews generally do not make aircraft configuration changes on landing until clear of the runway, and on takeoff, no configuration changes after the aircraft has been set up for takeoff.
That said, under very limited circumstances, mostly in light single and multi engine aircraft on rough dirt or gravel airstrips, I have applied flaps late in the takeoff; it's a bush technique to reduce flap damage during the takeoff roll, from rocks and gravel.  Applying flaps late in the takeoff has limited application.  Likewise, under similar circumstances landing, retracting flaps does put weight back on the gear and settle the airplane in on short, rough runways, especially where braking may not be effective due to the surface.  These aren't standard, and should be considered very carefully before trying, and then only by those who understand the practice and have experience with it.  There are a lot of things one can do wrong that can get one hurt, which is why the nearly universal standard practice is to not touch the flaps or configure until clear of the runway, on landing, and to be fully configured before every entering the runway to takeoff.


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Why are passenger seats not facing backward on an airplane?
In the event of a crash, facing backward sounds much safer:

Forces are applied on a  much larger surface (your whole back is in contact with the seats, instead of just a seatbelt and maybe the opposite seat)
Most passenger are already in position in case of an unpredicted crash.
Passenger without a seatbelt might have higher survival rate

Flying objects would maybe be more dangerous, but

What made designers do it this way ?
Is there any plane with such a configuration ?

Related: http://www.airspacemag.com/need-to-know/are-aft-facing-airplane-seats-safer-146695292/

you already have people that "get nauseous" when they sit backward on a train let alone a plane

Southwest used to have rear-facing seats against the bulkheads.  I rode (was stuck there) more than once.  This article suggests they were eliminated when they couldn't meet new safety requirements.  http://www.blogsouthwest.com/flashback-fridays-southwest-airlines’-interiors-over-years/

I've flown on Tu-134 aircraft in the USSR that had some seats facing backwards and some forward, to save space. Each bulkhead had backward facing seats, facing the next seat row. Allowed them to cram in a few more passengers (as the leg room wasn't twice that of a regular 2 rows of seats...).

You are absolutely correct, backward facing seats are safer.
But tradition and a subjective feeling of being treated better means that people will prefer to be seated facing forward. Some even claim that they develop motion sickness when sitting backwards. This can indeed be the case for some people in trains with their big windows, but much less so in aircraft.
Just note which seats will be occupied last in a train with lounge/mixed seating.
In order to cram the highest number of passengers into their planes, the airlines would need to convert all seats to the backward orientation, and you can be sure there will be some passengers who will complain. If passenger safety would be important to them, airlines could already use better seatbelts, like the 5-point-harnesses used in gliders and aerobatics planes, but they all use the minimum lap belt which is mandated and nothing more. Convenience and cost always win over safety.
Now I need to mention two caveats:

If passengers should be seated backwards, the industry needs to develop new seats. It will not be helpful to turn existing seats around - they would collapse at much lower loads than what a backward-sitting human can sustain. However, to fully support these higher loads, seats will be more heavy and possibly even the floor structure needs to be beefed up.
Backward-facing seating will only help in the fraction of cases where deceleration loads are too high for a forward-facing person, but low enough to make the crash survivable. If the plane flies into a mountain or ditches and sinks before people get out, the better seating will not help.

Some military transports have backward-facing seats. In some crashes, the survivability rate was seven times higher in those than in forward-facing seats. Early air travel also used mixed seating, e.g. on the Zeppelins or the Dornier Do-X. One airline to use mixed seating was Southwest Airlines with their "love seats", and business jets have mixed seating as standard. Thanks to @reirab for pointing out that United Business class also has some backward-facing seats.

Cabin of the Do-X. No seatbelts, seats not bolted to the floor. These were the days ...

Cabin of the Dassault Falcon 7X business jet.

Do military transports use beefed-up floors, then? (I'm assuming their backwards seats are just for safety, while business jets do it to let passengers face each other)

@cpast: Good question! Since the floor of military cargo planes is designed to handle cargo, a comparison with airliner floors is not straightforward. I would expect that the seats can carry higher loads, but I have no data on this.

i think the Grumman G2, which is used for COD, has backwards facing seats, given that it has a tailhook and is launched from a catapult. I found some pictures of the interior, but not that highlight the orientation.

@rbp I vaguely recall seeing interior shots of COD aircraft that have rear-facing seats, but I can't scare up any photos at the moment. I also recall them having either 4 or 5 point harness belts (to ensure the passengers stay upright during launch / recovery).

In British Airways' most common configuration of business class, half the seats are rear-facing -- example.

@GregHendershott: Thanks for the information, this is totally new to me. I wonder which seats are taken first.

I've heard some prefer rear-facing because with front-facing your head is lower than feet when lying flat (due to aircraft angle of attack); rear-facing elevates your head. O_o Also supposedly rear-facing is an impressive view out the window on take-off. :)

@PeterKämpf The same is true of United Airlines' business class.

I have a problem with your first statements and the contrast with the linked soutwest blog: When Southwest introduced the 737-700 in January 1998, new federal safety regulations doomed the lounge areas.  No rear-facing seats could meet this new safety requirement, and the -700s were delivered with all forward facing seating. So, it's tradition or safety requirements? - Also, if it is tradition, wouldn't this be contrary to the photos you posted showing old cabins with mixed seating?

@Federico: I would not automatically assume that the FAA bureaucrats know exactly what is safe. Their regulations simply do not reflect the insights from studies like the Aircraft Crash Survival Design Guide or the evidence from crashes with mixed seating.

And I would not assume it is because of tradition, since traditionally there was mixed seating.

The C-5 has a passenger deck above the cargo hold aft of the wing box with seating for about 70 passengers.  All seats face rearward.  The only windows are in the exit doors.  Not the most luxurious accommodation, but it's better than passenger pallets in the main cargo hold.

"Just note which seats will be occupied last in a train with lounge/mixed seating." British trains have half the seats facing each way (typically, all seats face towards the midpoint of the carriage). I honestly can't say I've noticed any particular phenomenon of forward-facing seats filling first, even when the carriage is very lightly occupied.

Using 3-point or better passenger restraints is not trivially easy; as described in §2.6.5 of the Kegworth crash report it requires significant seat or cabin redesign (enough to require the co-operation of the manufacturer) and also may obstruct movement over the seats in cases where an aisle is blocked.

It seems that backwarding seat make passenger feel not good.
However, some airlines offer backward seat porducts in business and first class
For example:
United B747 & B777 Business class

Etihad A380 business and first class

Morover, All(4) types of widebodies in BA do offer Business backward seats , its whole subsidiary Openskies also offer business backward seats in their b757

If anyone in business class doesn't like a backward facing seat, I'd be willing to swap with them...

When I've flown southwest the seats in front of the exits were rear-facing.  They were quite uncomfortable when the plane was pitched upward, but nice when the plane was pitched downward.

This is actually somewhat common on GA aircraft. The Beech Bonanza and Piper Saratoga, Meridian, Matrix, and Seneca have "Club Seating" (as well as I'm sure other 6 place planes I don't know about)
As seen here in this Saratoga

I don't know if this is a safety thing or a design matter but it does fall under your question Is there any plane with such a configuration?

I think one more reason is the pitch of the aircraft during climb and descent phases, and the take off acceleration. During take-off and early climb, as acceleration (almost 0.3g for an A320) slams the passenger into the seat back, it would be very uncomfortable to seat backward with only a belt retaining the pax (a 80kg passenger will be subjected to a force of 240N, which corresponds to the load of 24 bottles of water, distributed on all the back in one case, and only the surface of the belt with a backward seat). Moreover the pitch during initial climb can reach 16° (A320) and remains between 10 and 5° during all climb phase. It is oubvious that sitting forward is better (I mean, more comfortable) in those conditions. During descent and landing, pitch is not less than -3° (A320).
Regarding images with lounge in cabin, I dont know if passengers have to go back to their seat during take-off and landing, for safety reasons.

The VC-10s used by the RAF had all the seats facing backward.

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text:
How does SOFIA cope with open hull?
On the SOFIA aircraft, the aircraft is designed to fly at very high altitudes (higher than what can be achieved by regular 747), with a cargo door open.
https://upload.wikimedia.org/wikipedia/commons/8/8a/SOFIA_ED10-0182-01_full.jpg
I know that oxygen mask are a standard feature for most airliners but:

How is the aircraft designed to handle this situation ?
What about redundancy ?
If there is supplemental oxygen, is it really without adverse effects ?

pic credits wikimedia

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text:
How much efficiency gain can be attributed to aircraft handling?
Since the beginning of commercial jets, constructors thrived to make planes more fuel efficient. I assume most gains since then come from cleaner aerodynamic design, weight savings, and engine design.
However, these are not the only factors to take into account.
For example, we have also became better at predicting meteo, or transmitting live information about airstreams so that planes can spend more time in the most favorable zones.
Traffic control can also improve fuel efficiency by optimizing the landings of planes or reducing taxi/idle time.
How much of the gain in efficiency can be attributed to this factors ?
Another way to phrase it would be: How much more fuel efficient would it be to fly an old plane with modern avionics today?

I do not believe this question is too broad, it's simply a high level question that requires a broad answer.

That's the definition of too broad @JayCarr...

How shall I edit the question to make it more suitable ?

While a good answer might include lots of informations to explain the efficiency gain, an acceptable answer could be a one liner: "We gain ± n% source:[url]"

@GdD I think I more meant broad in the "general, high level" sense.  Not the "detailed, multi faceted, should have written a book" sense.  High level overview questions, like this, are fine.

I don't think this is all that broad at all. It can be answered fairly succinctly. Additionally I think it's a really good question that hasn't been posed here before.

In a paper written in 2010 several factors are laid out in addition to aircraft design that have improved fuel efficiency and cost over the years and possible improvements in the future. Several areas were highlighted.
Routing
Opening up more airspace by more efficient usage increases efficiency.

It is estimated that up to 8% of all aviation fuel is wasted as a result of the inefficient routes aircraft have to fly.

The paper cites flight management systems on newer aircraft as contributing toward efficiencies in routing.

On certain oceanic routes, flight control computers are automatically plotting their own, most efficient routings with some impressive results. One airline, for example, has been working with Australian air traffic management to save almost 10 million litres of jet fuel and 772 hours of f light time in five years. It does this by exploiting the jet streams and tailwinds in the Indian Ocean.

The following charts show projected savings for implementation of NextGen air traffic systems in the US and SESAR in Europe.

Continuous climb
Being able to climb continuously to cruise altitude can save on fuel.

A number of airports and airlines are trialling the use of so-called ‘green departures’, allowing pilots to take-off and climb to the optimal cruising altitude in one smooth, continuous ascent. This is   in contrast to the traditional method of climbing to the cruising altitude in several steps. By using this new departure method at one airport alone, some 10,000 tonnes of fuel and 32,000 tonnes of carbon dioxide were saved in one year alone.

Continuous descent
Similarly to climbing, fuel can be saved by avoiding stepped approaches.

In trials, fuel savings of up to 40% during the approach phase have been demonstrated. This equates to between   CDO [continuous descent operations] vs stepped approach 50 and 150kg of fuel depending on the level at which CDO is commenced and the aircraft type. Up to 150,000 tonnes of fuel a year, or 500,000 tonnes of CO2, could be saved in Europe alone if CDO approaches were more widely adopted.

Satellite based approaches and departures
Using RNAV procedures help expedite aircraft through the airport environment.

Using satellite-based and on-board precision navigation systems such as “Area Navigation” and “Required Navigation Performance” capabilities allows ANSPs [Air Navigation Service Providers] to re-design airspace and procedures so aircraft can fly automatic fuel-saving routes into and out of the busiest airports in the world. These new departure routes have reduced departure delays of more than 2.5 minutes per flight at one airport since their introduction. Annual fuel savings are estimated at $34 million, with cumulative savings of 105 million from 2006 through 2008.

Ground handling
Fuel is saved by using single engine taxi or tugs where appropriate.
Another place more efficiency can be garnered is by installing ground electrical power to reduce APU usage.

studies suggest that up to 85% of APU use can be reduced if groundbased electrical power systems are available, cutting the fuel bill, per gate, by $100,000 a year.

Reducing delays
Another costly problem is flight delays. Aircraft end up sitting on the ramp or taxiway burning fuel waiting for clearances.

In the United States alone, the cost of burning fuel on the ground as a result of delays to the airline schedule amounted to over $5 billion in 2008 alone.

Conclusion
The following graph represents the potential fuel savings projected into the future broken down by technological improvements in aircraft design and fuels, operational issues such as ground handling and delays, and infrastructure such as airspace availability and routing. Of course any future projection is speculative.


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What is the chime that can be heard roughly 30 seconds after takeoff?
This may be confirmation bias on my part, this might be impatient passengers pressing the call button, and it might be me imagining things, but it's one of those I've wondered about. I'll try to describe as best I can.

Of all the times I've flown recently on an airliner, of which most of the time it's a short haul carrier often flying smaller Boeing/Airbus aircraft*, I have been aware that there is an audible chime which can be heard throughout the cabin (ie, not only when sat in the vicinity of crew seats) very soon (30-45 seconds) after take-off.
It is heard well before the passenger seat-belt signs are switched off.
Can anyone describe what I am hearing and what its purpose is? Who is this a signal for, and who is sending the signal?

* the two times I've flown on an Embraer I don't recall hearing it

It's signalled from the cockpit (the cabin chime) to let the cabin crew know it's OK to move around.  If you watch, this happens before the seat belt sign goes off which at the earliest, is at 10,000 feet.

You may be right that it is the point that the cabin crew may move around. It is definately before the passengers are allowed to do so.

See Cabin chimes and overhead panel lights for the lights associated with the chimes.

This is the right answer.  @Jamiec the reason you didn't see this is the Embraers is the "sterile cockpit light" is used to signal the flight attendant that they can stand up and move around.  Next time you're on one, look for the blue light just above the cockpit door, and when what happens in the cabin when the crew turns it off after passing through 10k feet.

No way the cabin crew is allowed to move just 30 seconds after takeoff. That is a different chime, indeed around 10,000 ft. Why was this the accepted answer instead of @K48 ‘s one?

To be precise, on the Airbus, the chime comes on after the emergency exit signs are turned off. In the Airbus Normal Procedures, the Emer Exit Signs are set to Auto, meaning the Signs would turn off after the LG are up and locked.
So it is not a chime to inform the cabin crew that the gears are up and locked, but a chime that sounds when the Emer Exit Signs are turned off.

It informs cabin crew that a positive climb on takeoff has been achieved and that there are no immediate dangers/threats they need to be alert to from the takeoff role, as this part of the flight is most vulnerable. The second chime shortly after allows them to mobilise around the galley, and cabin in prep for service just ahead of the seat belt signs being turned off, hence an announcement to remind passengers that the seat belts signs are still turned on is announced so people don't start getting up for the loo because they now see the crew moving around.

Actually. The chime is to signify the exit signs are now shut off.  But also, nobody mentioned this, but it signifies the exit sign lights are physically recharging their batteries

There is no automatic chime on a Boeing! Any chimes heard immediately after takeoff will likely be the cabin crew using this “dead” time (unproductive time prior to the extinguishing of the seat belt light by the captain) to start coordinating service details.

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text:
How does a horizontal tailplane stabilize a cambered airfoil aircraft?
The picture below shows the location and movement of centre of pressure when angle of attack is increased. Consider the aircraft is trimmed at the second airfoil in the picture. Thus the horizontal stabilizer has to provide a downforce to trim the aircraft. Now if the angle of attack increases, the centre of pressure moves towards the .25c point thus reducing the moment. If the stabilizer is still providing downforce, then the aircraft is unstable. How does the stabilizer give positive static stability?

What you show is a 2D view of the center of pressure. But the center is not the same on the whole wingspan and the wing can be swept. What will count is the overall torque for the lifting surfaces, the whole wings and the fuselage too (aerodynamic center). See here. That said, the inversion you describe exists, just think of aerobatics.

See also this answer for an explanation. When the AoA on the wing changes, it does equally so at the tail, so the downforce is diminished as AoA goes up.

@PeterKämpf, Even if the downforce is diminished as the AoA goes up, it still doesn't provide the pitch down moment necessary for positive stability. The above figure is true for a rectangular wing with no washout, sweep etc. right? In the pages you have linked, there is a comment that says that the tailplane does not need to produce a downforce in a conventional tail. How will the aircraft be stable then ?

@GokulJ Yes it does. Did you read the first paragraph of this answer? What is it you don't get? The relative lift change at the tail is bigger, and the direction of lift at the tail is secondary.

I'm sorry sir but I don't get how having less lift per area gives a higher lift change in the tail than in the wing


text:
Mode S Transmission from Boeing 737 Integrated Standby Flight Display (ISFD) data?
During a recent flight, while in communication with London control*, we received a query about our RVSM (Reduced Vertical Separation Minima) compliance. Both the Captain's and First Officer's altimeters were set to STD - and within 20ft of each other - which left us curious as to why the controller asked about our RVSM capability. However, during a subsequent check, we noticed that our Integrated Standby Flight Display (ISFD) was not set to STD, it had been inadvertently overlooked after departure.
Does the Integrated Standby Flight Display (ISFD) on our aircraft (Boeing 737) transmit information via Mode S to air traffic controllers? I couldn't find any reference to this in the Boeing Flight Crew Operations Manual (FCOM). Could the controller's query about RVSM compliance be related to our ISFD oversight, or are these two issues unrelated?
*Just feel that it is important to note that they are exceptionally good controllers.

The Mode S transponder transmits uncorrected pressure altitude (see e.g. here). Your QNH/STD setting shouldn't have any impact on the transmission.


text:
Are Pitch Attitude and %N1 values for single-engine flight provided in the Boeing 737 Documents, or are pilots required to rely on “rules-of-thumb”?
In the Boeing 737-700/800 Quick Reference Handbook (QRH), the section titled "Performance Inflight - QRH" provides Pitch Attitude and %N1 values for various weights and pressure altitudes during Flight with Unreliable Airspeed. These values are useful to be aware of for manual flight, as pitch and power datums, especially in simulator events.
However, I couldn't find any listed values specifically for single-engine flight in Boeing’s 737 reference materials. I am curious to know if Boeing includes Pitch Attitude and %N1 values for single-engine ops in their manuals?
Alternatively, do pilots have to rely solely on "rules of thumb”?
For example:

For Single Engine add 15% N1 + 5% N1 in turns?
b737.org.uk

@757toga thanks, definitely sounds reasonable. What would their “thinking” be regarding single engine ops - it seems a little strange to have no rough figures for N1% settings when AT not available.

@757toga  thank you.


text:
Origins and Efficiency of the Phonetic Alphabet in Aviation
I’m curious about the history and practicality of the phonetic alphabet (Alpha, Bravo, Charlie, Delta, Echo, etc.) used in aviation communication. While I recognise its universal application in various fields like police services, my question primarily focuses on its aviation aspects:

Origins: When and how did the phonetic alphabet become incorporated into aviation communication? Was there a particular event or need that prompted its adoption?
Word Length and Efficiency: I’ve noticed that some phonetic representations, especially in aircraft call signs like “60NU” (Six Zero November Uniform), can be quite lengthy in terms of syllables. Has there been any industry discussion or attempt to streamline the phonetic alphabet to reduce the number of syllables to one or two per word, for efficiency in communication?

Efficiency is not important. Reducing misunderstandings is. It doesn't matter if it takes you .2 seconds longer to say a few more syllables, it does matter if you have to repeat your entire message because someone misheard a letter

@60levelchange efficiency surely remains an important aspect in comms? Conciseness is part of effective communication. A desire for efficiency has led to the shortening of callsigns from G-ABCD to G-CD, as well as the use of airline callsigns.

It's not important RELATIVE TO reducing misunderstandings (which themselves will lead to less efficiency)

In 1944 english was chosen as the universal aviation language to simplify communications in the sky. Since not everyone is a native english speaker you have issues of accents when using a common language. Not long after in the 50's NATO realized the need for universal pronunciation of letters and numbers and the first phoetic alphabets were proposed and adopted. To ensure smooth communication a set of words was chosen that represent the numbers 1-9 and letters A-Z were chosen that were distinctly pronounceable through many accents and would not be confused with native words in those languages. The goal of the phonetic alphabet is not syllable efficiency but universal pronunciation for which it is well optimized.

It's an offshoot of military communication protocols, the military being the first institution to adopt large scale use of two-way voice radio. It became obvious early on with carbon microphones, static and audio distortion that a way needed to be found to make letters easier to discern, so two or three syllable words that were phonetically discernable and unique, even when garbled or buried in static, were adopted that could be linked to the applicable letter of the alphabet via that word's first letter.
It's evolved over time to the current Alpha Bravo Charlie standard in aviation, largely the result of an ICAO standardization project implemented after WW2 (during WW2, the US Army used a different phonetic alphabet, starting with Able, Baker, etc).
Other institutions use other variations on the same concept, such as the use of first names to represent letters in policing radio comms.
With numbers, it's mainly a practice of exaggerating the pronunciation of numbers, except for the number nine, which is impossible to discern from the number five if the audio is bad, hence "niner".
In Europe and North America at least, the audio quality of ATC comms has become good enough that you could get away without the phonetic alphabet much of the time, and you often hear 9 just said as "nine" not "niner", but comms audio in other places may not be as good, or you may be talking on HF, which is like ordering a burger at a drive thru. Add to that that need to mitigate workload/fatigue related errors even with good audio, so the protocols are still necessary.
So to answer Question 2, the ICAO phonetic alphabet is the result of a lot of research and evolution over the years to find the ideal phonetic alphabet that is efficient, pronunciation wise, and effective at the same time.


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What is the purpose of the second wifi network on board (some?) B787 aircraft?
I recently flew for the first time on a Boeing 787-8 Dreamliner, registered A7-BCY. This aircraft had the universal symbol for wifi externally displayed beside the entrance door, and as wifi on an aircraft was another first for me I was keen to try it out. Which I did, without much fanfare.
The wifi used to provide internet access was named along the lines of "OneAir", which is consistent with the name of that airline's connectivity/On-board entertainment services.
What intrigued me was that there was a second wifi network available (albeit secured from public access) which was named with the aircraft's registration "A7-BCY" in the SSID of the network.
What is the purpose of this network? What data would you be able to access/see if you were to connect to it using a laptop (for example)?
(Note: I know there have been some questions here and on computer security regarding the vulnerability of modern airliners to hackers - I'm less interested in "doomsday scenarios" with this technology, and more interested on what legitimate uses there are of this network)

http://www.boeing.com/commercial/aeromagazine/articles/qtr_01_09/pdfs/AERO_Q109_article05.pdf has some general information.  I imagine it's used for maintenance.

If it's just used for maintenance, which is not inconceivable, I wonder why they wouldnt just shut it down when in operational mode. Seems like a waste of power. Also, if used for maintenance only it seems like a waste of weight to cart it round with you the whole time when it could be an external unit when needed.

The power required to run a WiFi channel inside an aircraft fuselage, especially if nothing is connected to that network, is negligible.

I would say the PDF found by @sbooth pretty well answers this question. It covers the "What is the purpose of this network?" question pretty well, and, by inference, the "What data would you be able to access/see" portion, as well.

Aside from it being on, which is OK, I'm surprised they broadcast the SSID at all, seems like a security issue to me...

Maybe it's used to stream the kind of info that's usually recorded on the airplane's "black boxes" so they don't have to go looking for them if the plane disappears.

@ron Broadcasting the SSID or not means nothing for security.  It's trivial (I have an app on my phone) to find networks not broadcasting the SSID.

@HowardMiller stream it to where? The passengers on the plane?

It's most likely the same wifi router,, so it's not adding any weight or power usage unless something is using it. My home router allows me to have two distinct  SSID's with seperate logins and passwords. The second ID might be used by the crew in-flight for EFB functions

@JustSid Dish Network

@sbooth: The OP says they detected a base station with the SSID being the RN of the plane. This means the aircraft is not a client in this case (there is no SSID broadcast for a client), but a base (an access point in the lingo). The link to E-enabled shouldn't be related as the aircraft would be a client, not a base. It could a protected access to the "same" W-LAN that the one for passengers, but with increased privileges. I guess the two AP are on different frequencies, one on 2.4 GHz, the other on 5 GHz.

@TomMcW If it were actually a maintenance network (which it apparently isn't,) then I'd be a bit surprised if it were actually two BSS's on the same hardware. If I were designing an airplane with both a maintenance and a passenger Wi-Fi network, I'd air gap them. And probably also shut down the maintenance network when it isn't in use. For its apparent actual use case (cabin crew iPads, though,) then, yeah, you're right that it's likely the sharing access points.

I think it has to do with the use of IPads by mainly the cabin crew, according to this article:

It also helps to identify customers quickly, and can be used to alert other staff of a passenger’s special meal or service requirements.

In order to have to crew communicating to each other via the IPad they need to be connected somehow, so I guess that the network you see is just linked to a locally hosted server such that all the IPads can acces the same data even if a crew member makes a change.


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How are airliners secured on the ground during hurricanes?
Say a major hurricane is expected to hit. Given that the weather is known beforehand, what can be done to avoid damage to the airliners sitting on the tarmac?
Fly every plane away - not very economical, considering that commercial operations cease at the last minute.
Move all planes into a hangar - a Boeing 747 takes a huge hangar to park. It may be possible to move all GA planes inside, but certainly not airliners.
Planes are designed to interact with winds; the wings is meant to generate lift, and the sideway forces on a vertical stabilizer is meant to swing a plane around. What procedures would minimize the stress put in the airframe by the 100+km/h winds?

I have seen tie-downs, but this still happens: https://www.youtube.com/watch?v=piPoF-3ynZk

While it may not seem economical to fly the airplanes away, it is far more economical than scrapping a few of them due to damage done by a hurricane...

Having airliners sit on the ground is not very economical either, so airlines will generally try to have them in the air regardless of hurricanes at particular airports.

The move them. You should have seen Newark before the hurricane. Totally deserted. Normally the place is packed with airliners parking overnight.

100 KPH winds are strong, but a 747's Vr is well over 250 KPH. Properly position the flaps and you'll need considerably more airspeed (in the right direction) over the wings to lift the entire plane.

@dotancohen 100 km/h crosswind (or even tail wind) is very strong (you have no guarantee where the wind is blowing from)

@kevin: Strong, sure, but not enough to flip the plane. It sure will bounce around quite a bit, though!

@dotancohen: Flip, no; damage (potentially quite severely), yes.  Even strong winds on their own can cause flight-control damage if they're coming from the sides or rear, and the strong winds in a hurricane tend to be carrying a lot of stuff with them besides just air.

Looking at the hurricane manuals for a few airports, evacuating flyable aircraft to a safer location tends to be the most common choice.
From the Boca Raton Airport:

Effective with the issuance of a Storm WARNING and/or Hurricane WARNING:
Relocate flyable aircraft to other geographical areas, place under suitable shelter, or secure as appropriate.

Also, from the Miami International Airport:

Upon receipt of a Hurricane Warning, Airside Operations will:

Initiate evacuation of all flyable aircraft and post Airfield Operations Senior Agents and Agents in detail to insure compliance and report results.

My dad flies GA, and during the last hurricane we had up here in the NE, I believe his air club offered some nice getaway deals. I forget exactly what it was, but something along the lines of "if you fly it to safety, room/board costs are on you, but the engine hours are free."

Out of curiously I once looked up what had become of the DC-3 that was owned by John Travolta. Turns out it was in Opa Locka when it was hit by hurricane Wilma. It was written off.

@yshavit you had me very confused. I'm thinking, "A Hurricane in Nebraska?!" I'm assuming by NE you mean New England? Didn't notice "the NE"

Must mean "the northeast". You don't say "the New England".

Oh, yeah, the northeast. (Though, as it happens, also New England specifically) :)

@TomMcW We've had a Tropical Storm Warning in Tennessee before. That was interesting, to say the least. It was Katrina, of course. It was literally still a tropical storm nearly 500 miles inland. It was still a Category 1 Hurricane with an eyewall nearly 300 miles inland.

@TomMcW: Archived version of that page which still shows the actual picture.

Regarding the comment about aircraft being designed to handle high winds, presumably because they can fly > 350 knots; beside the direction of the wind being unpredictable and gusty, airplanes are not designed to deal gracefully with things being thrown at them——you know like cars, sheds and cows——or being tipped such that fuel laden wings and tanks, or exterior sensors and gear, bang the ground.  I'm pretty sure flying them a few hundred miles away is less expensive than losing equipment that costs \$25m-\$60m per, even with shuttling around crews, parts, fuel, etc.  I suspect tarmac tie-downs, like those you might see for smaller planes, are not anchored with > 100 knot winds in mind, and how often are they checked?

It's a valid point.


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Where Can I Find the Master Minimum Equipment List for the A320 Series (UK / EUR)?
I'm currently trying to locate the Master Minimum equipment list for an Airbus 321.
I have the FAA (US) version of this document, but I'd like a copy of the UK or EUR version if possible.

The Master Minimum Equipment List page on CAA doesn't contain a link to the any airbus series
http://www.caa.co.uk/application.aspx?catid=33&pagetype=65&appid=11&mode=list&type=sercat&id=16
I found a page on the flight safety committee that contains 5 historical links to lists for the A320 series aircraft. But all of these links are dead.
http://www.ukfsc.co.uk/information/caa-notifications/275-caa-master-minimum-equipment-list-amendements-mmel

Can anyone point me in the right direction as to where I can find this document? I would prefer the revision that was current in September 2013 for UK and Europe.

Send an email to Airbus?

@ratchetfreak thanks... I've sent an email to CAA asking for this information, but in my experience they aren't very timely at responding..  I did check the Airbus website, but unless I'm missing something obvious, they don't have a contact email address on there. Airbus Contact Page

The FAA has a list here that includes Rev 22a, dated 2/7/2013 (Feb 7th).

The latest revision of the MMEL for the A320 family was approved a month ago by EASA. The EASA website only contains a PDF document with the MMEL approval signature for Airbus aircraft.  For some manufacturers (e.g. Cessna) the full MMEL is included on the EASA website.
The information page on MMELs says:

Each MMEL document is the property of the (S)TC holder, and only made
  available here in full if requested by the (S)TC holder. If a full
  copy is not available here then please contact the (S)TC Holder to get
  a copy of the document in question.

So you have to contact Airbus in this case.


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Why is vertical take-off restricted to lighter weight aircraft?
Vertical take off is a big advantage, but why it is only limited to low weight?
For example, fixed wing like An225 can lift more than hundred tonnes of payload, while the biggest helicopter is 25 tonnes. Why there is no effort to build a helicopter can lift 50 - 100 tonnes. What limits them? Economic or technical problem? I guess it is a technical problem because even the heaviest helicopter is just an experiment, but what is it?

To be fair, the biggest helicopter lifted over 44 tons. On the other hand, An-225 lifted over 250 t :)

If we want to stretch the definition of aircraft a bit, the first stage of the Saturn V lifted well over 3000 tons, vertically.  Which if you think about it does tend to shed light on the relative efficiency of vertical takeoff.

Essentially it comes down to supersonic rotor tips
With a plane, in theory you can make it pretty much as big as you like - as long as you have strong/light enough materials, and can keep adding power, an aeroplane design scales pretty well. Bigger wing = more lift. As long as you can make the wing bigger without it breaking, and as long as you can add enough power to overcome the extra drag, there aren't many fixed limits
With a helicopter, we're limited by the rotor tips: once they go supersonic, they cause a lot of problems.
So how does a helicopter produce lift? By using rotors to push air down within a kind of circle. To add more lift we can do (essentially) three things.

Make the rotor spin faster, so it pushes more air down in the circle it already uses. Obviously this makes the tips spin faster, so we can only do it to a certain extent. We've already hit this limit.
Make the rotor blades longer, so they push a bigger circle of air. Again, though, due to the nature of a circular blade, the outside of a blade is moving faster than the inside. For a certain rotor speed, there's a fixed limit to how large the blades can be. Again, we've already hit this limit
Add more blades, so there are more blades producing lift. This works to an extent (hence why smaller helicopters may have two rotor blades, but larger ones have 4, 5 or more. Again, though, this doesn't scale indefinitely - each rotor interferes with the next, you can't just keep adding more

There are other slight modifications we can make, such as the airfoil of the rotor, but they don't add significant gains
So, basically, we've hit the limit of what we can lift with a single rotor, The only real way to add more lift now is to add more rotors: doing that would be far less efficient than simply using an aeroplane.
Which brings me to the final point - helicopters are very inefficient and pretty slow... We simply don't need, except in a few niche circumstances, to carry more weight with them.

+1 for the final point. To perhaps be a little more clear, helicopters require thrust/weight ratios > 1. Airplanes do not. Even if we could put enough rotors and enough blades on it, the engines would have to produce a lot more power than those of an airplane carrying the same payload. Beyond a point, we'd have to start installing vertically-mounted GE90s or some such thing on them. It should also be noted that this limitation applies to all vertical takeoff (except lighter-than-air craft,) not just helicopters.

Aircraft require thrust to weight considerations, it's just not exactly proportionate in the same way

Yes, but they're usually closer to 0.25, not > 1.0. Much less engine power (and, therefore, much less fuel) for the same lift.

True, fuel burn becomes an issue too - I may add that into the answer

From physical point of view, isnt that power and force can be traded? E.g. a longer lever will have higher force for the same power?

@user2174870: From a theoretical viewpoint, yes. In reality levers add weight. And a lever also needs a support point, which is rather inconvenient for an aircraft.

I think that you forgot an option : add more rotors (like this)

@quentin see the penultimate paragraph, I did cover that - it's just usually unwieldy and becomes more sensible to just use a fixed wing: other than the Chinook, I'm not aware of a multi-rotor design that's actually accurate enough in terms fo flight characteristics for in-situ lifting... and if you can't pick the payload up from anywhere it happens to be but have to take it to an airfield, you may as well use a plane

Jon's answer is correct: "helicopters are very inefficient and pretty slow..."
but he misses one important point: helicopters are also incredibly fragile and delicate. Even a regular single-rotor helicopter flying is a miracle. It's been called: "10,000 spare parts flying in close formation."
You could add more rotors to gain more lift, the Chinook has two rotors which is part of why it can carry so much. The rear rotor adds complexity but also removes the need for a tail rotor, which is why it isn't quite 20,000 spare parts flying in close formation.  But even Chinooks are fragile compared to C-130s.
Adding engines adds redundancy in a plane. And remember that if you are in a plane and you lose an engine (or all engines) you can still fly or glide to a safe landing.
If you are in a rotorcraft and you lose your engine (either main rotor or tail) or really any single part of those 10,000 parts, then all you can do is pray and try an auto-rotation landing.  It gets worse the more rotors you add, not better. This is one of the limitations of rotorcraft and why you for the most part only see multiple rotors (4-, 6-, 8-) used in unmanned drones except for some very experimental vehicles.

A big advantage for the Chinook is that you can have a load center anywhere along the midline rather than only exactly below the rotor center. Very useful for a military transport with underslung cargo

"You could add more rotors to gain more lift" and yet the heaviest-lift chopper to go into production was a single rotor design.

The power required to maintain altitude at forward flight is lower than the power required to hover. As horizontal velocity ($V_x$) increases, the induced velocity ($V_i$) at the disc decreases, and induced power ($P_i$) decreases roughly equal to $\frac{1}{V_i}$. However, as $V_x$ increases, parasitic power ($P_p$) increases proportional to $V^3$. Profile power ($P_0$ - the power needed to maintain rotor speed) remains roughly constant. If you chart all of these together with Required Power on the y-axis and forward velocity on the x-axis, you end up with something that looks like this:

What this means is that there's an optimum forward speed at which your power is minimized. This is important, because the max rate of climb (and lifting power) is determined by your available power $$P_{av} = P_{tot} - P_i - P_p - P_0$$. If you minimize the power needed to keep the aircraft flying, you have more net power for lifting other things (and for heavy helicopters, they may not be able to take off vertically at all). For this reason, the max rate of climb will always be at some forward speed, and by definition, the max lifting capacity will also be at some forward velocity. A practical demonstration of this principle is that large cargo lifters like the sky crane and the chinook always take off with some forward velocity component. This is why.

good answer, do you have a source for the image?

In addition to the power-to-weight ratio - already aptly described above - I bluntly strike at the economic reasons and cite that bean-counters look at the cost-per-takeoff and shake their head.

Yes.  Even assuming that you could solve all the technical problems of a really heavy-lift helicopter, where's the economic demand for them?  What can you do enough of cheaper with a big helicopter to pay off the development & operating costs?  Even where there is demand for heavy lift, it seems that it can be met better with airships: http://www.bloomberg.com/news/articles/2013-06-14/blimps-morph-into-haulers-as-airship-maker-sees-cargo-revolution

This seems to echo the current top-voted answer's final statement: "We simply don't need, except in a few niche circumstances, to carry more weight with [helicopters]." We tend to prefer answers that add something substantial that has not already been written in another answer to the question; can you edit your answer so that it does so?


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Can a small plane be floated off of an aircraft carrier?
Would it be possible to float a small plane (say a Cessna 150) off of the deck of an aircraft carrier without the Cessna achieving any forward motion with respect to the aircraft carrier? If not a Cessna 150, is there a standard GA aircraft that could do it?
EDIT:  Per a comment from ratchet freak: the takeoff would be done via lift from the wings, not via an engine duct (VTOL), like a Harrier.

excluding VTOL capable planes I guess

Given the right airplane, you can float it even off a submarine. The Focke-Achgelis 330 [http://en.wikipedia.org/wiki/Focke-Achgelis_Fa_330] was designed to be towed by a submarine, much like a kite, to give it a wider field of view.

@PeterKämpf That might make a good answer if you expanded it out.

Yay! Fuel-economic takeoffs! :D

Given that small aircraft have taken off, so to speak, on land during severe wind conditions, it's more interesting (as the answers suggest) to see what you can do sans storm :-)

@PeterKämpf link to wikipedia is corrupted - has extra ]. It should be http://en.wikipedia.org/wiki/Focke-Achgelis_Fa_330

Keep in mind that the thrust generated by the propeller must exceed the force of the the wind. Otherwise, the aircraft is just going to be blown backwards.

@user3344003: If the thrust from the propeller didn't exceed the force of the wind needed to generate enough lift to lift off, the plane wouldn't be able to fly anyway.

As far as I'm aware this has never been attempted (and I doubt the Navy would let me try - though I'm certainly game if they are!), but:

A Piper J3 Cub stalls at 38mph (33kts).
The USS Enterprise (CVN-65) had a maximum speed of 38.7mph (33.6kts).

So, given a lightly loaded cub and an Enterprise-class carrier at flank speed, it is at least theoretically possible to float that aircraft off the deck (if it were lightly loaded, and thus had a lower stall speed).
Achieving zero relative motion would require a little more effort - specifically you'd have to seek out a headwind, and the Cub would have to be operating at an airspeed above stall, but still pretty close to it.
For example, if the Enterprise were making 30 knots and steaming into a 10 knot headwind the relative wind on deck would be 40 knots. That's enough to lift the Cub, but as soon as it was in the air it would need to pull itself along (via its engine) to maintain that speed. The Cub would have zero relative motion above the deck at an airspeed somewhere around 40 knots).

As a (slightly ridiculous) bonus: The Belite Superlite has a stall speed of 28 mph (25kts). It would be theoretically possible to fly such a plane like a kite from an aircraft carrier. Its advertised takeoff and landing distances (both ~500ft) are also such that it could operate from a carrier deck without need for the catapult to launch or arresting wire to stop.

Any idea if the air coming over the bow of the ship would affect trying to float?

It's also worth noting that relatively large planes (B-25s and the C-130) have been launched from (and landed on) carriers - None of these would be able to achieve zero relative motion though - their minimum flying speeds are well above 33 knots.

Here's a civilian aircraft taking off from a (decommissioned) carrier in a 100 ft or so.

@voretaq7 are those stall speeds with full flaps/slats for the J3 cub?

@NickT those stunt planes have almost a 1/1 thrust to weight ratio (look at that near vertical climb) so I'm not surprised that thing can take off there

It has not been attempted from a carrier, but it has been attempted from a merchant ship with sufficiently flat deck. See http://www.youtube.com/watch?v=pUdzVnZBaoY. The ship was not sailing fast enough for the plane to lift off from stationary though.

@shortstheory Re: the cub stall speeds, I believe that number is for the plain Cub (which has no flaps) - I'm not sure what the speeds are for a Super Cub (with flaps and other goodies).

@voretaq7: oh thanks, I think I incorrectly assumed it was for a super cub.

@JanHudec - great video, note also that the ship was at 9 knots with a headwind of around 5-6 knots, for an effective wind of around 15 knots. As mentioned, the plane wasn't stationary, but if the ship was even doing 20 knots it should be possible

A Cessna 150 can take off at 48 knots rotation speed. An aircraft carrier can do 30 knots, and an 18 knot headwind is certainly possible at sea. That puts a 48 knot wind over the wings, enough for the Cessna to take off. The engine would counteract drag, allowing the plane to "float" off the deck.
If you have any doubt about this being possible, just watch what happens if you don't tie down your plane properly.
http://www.youtube.com/watch?v=IPOtDPHjW-Y

What you say is not only possible, but it has been used to land a plane on a much smaller (and a bit faster) ship than an aircraft carrier.
Try to search in the web for the video "Extreme small plane landing on a ship at sea".
Here is a working link at the moment: http://www.youtube.com/watch?v=pUdzVnZBaoY

It wasn't even faster ;-) that ship was at approx 9 knots.

It all depends on the minimum flight speed and the ship's maximum speed. Even with the slow speed of submarines, floating a flying machine is possible if the machine has been designed that way.
In order to increase the observation radius of submarines (typically only 5nm), an unpowered gyrocopter was designed in 1941 to lift an observer 500 ft up into the air to increase the observation radius to 25 nm. Gyrocopters are known for their low flying speeds, and this one, the Focke-Achgelis 330, was designed to be folded away in a watertight container and to be stored on the deck of the submarine. It could be assembled and de-assembled within 20 minutes and was then towed behind the submarine like a kite. The minimum flight speed was only 27 km/h (14.5 its), just enough for a submarine going at full speed and a little wind.

The forward speed of a carrier might not be the listed max speed, if heading into a strong headwind.
However, if the relative wind is great enough, the aircraft wings would create more lift than the weight of the plane.
There are of course issues like structure induced turbulence which might adversely impact handling.  And if near equilibrium, the aircraft may never fly beyond the bow.
If planning on this type of operation,  would look for a Vx (adjected for engine off or idle) relative wind to allow a reasonable climb (if you call the ascent that) to clear the structure.  Also, I would plan on factoring in the ground effect of working off the carrier deck, and then having that effect reduced as altitude was gained.
Finally, from practical experience, I would expect the aircraft to become airborne prior to the stall speed, due to ground effect, probably a lighter that gross weight loading, and lots of other factors.

The aerodynamic drag on an aircraft carrier from sailing into a headwind is utterly negligible compared to the hydrodynamic drag from moving through the water at 30+ knots.

I took off once at Wellington airport, NZ in a Gulfstream Cheetah in winds gusting to 55kt. When I opened the throttle it lifted off with no forward ground speed at all. All that is required is a vessel fast enough to provide a steady 45kt wind speed over the decks.
Some designs of fast ferries like the INCAT wavepiercing catamarans would be ideally suited as some form of aircraft carriers like this.


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How do pilots manage to keep aircraft on centerline during taxi?
Cockpits usually seem to have windows that are high up and it doesn't seem from pictures that they would allow a pilot to look down so easily while sitting in their seat. Please correct me if that is wrong because that is the basic assumption of this question.
Then how do pilots know where the centerline is on the taxiway and runway? They always seem to taxi with nose wheel almost always on that line even when turning. Are there some instruments that help them? Or its the same logic as in a car as to how you find out where the lines are?

Related: http://aviation.stackexchange.com/questions/3316/visibility-from-a-cockpit

In general, on big iron, position so that the centre line, if projected into the cockpit, appears to run down the inside of of your right left if in the left seat, vice-versa for the FO.

Wasn't there some sort of WWII aircraft whose nose was slanted upwards while at rest on the ground and the pilot literally could not see the runway at all during takeoff? I recall some documentary or other that mentioned this.

@MichaelMartinez - they're called tail-draggers, and are still alive and well (though less common)

very good question, like if the captain has given full throttle and the centre was miss alligned, it could be fatal, the answers seem to tell that this is not a big problem its something intuitive.

Sometimes they don't!

Aligning yourself in the middle of a straight taxiway isn't a problem. On the 747, if you put yourself in the middle of the taxiway as you see it beyond the nose, you'll be in good shape. If you're the captain, you can hold yourself a little left of the perceived center, but not much, because you're looking at from an eye level over 30 feet up as I remember. If the taxiway has those little metal reflectors sticking up on the centerline, you can play games trying to keep one of the nose wheels hitting them. You can just barely feel going across them. Or, since the nose gear is two wheels, you can play with getting the reflectors between the two wheels if you're really bored.
Turns on a taxiway present a bit more of a challenge insofar as keeping the center line because the nose gear is behind you, but one develops a feel for it. A 90 degree turn from a narrow taxiway onto another narrow taxiway is a little challenging, especially if the taxiway you're on T's into the one you're turning onto. All you'll see out the front window as you start the turn is grass. Also, on narrow taxiways, you can't see the taxiway out the side window from your normal sitting position.
If you need to do a 180 on a runway, you need to remember that when you first get yourself on, say, the right side of the runway, the nose gear isn't there yet, and the wing and body gear have even further to go.  You have to run yourself along the side of the runway a sufficient distance so that the airplane is aligned with the edge of the runway before you start your turn.
Personally, I found taxiing a 747 one of the more fun things in life.

I've always wanted to drive a city bus, just for the challenge. You've found me a new challenge in life! Sadly, for a software developer, it seems even less likely to become true than the bus dream. At least MiGs are not out of the question.

@dotancohen - but taxiing a small plane is not out of the question as a software developer :)

@warren And software development (or a sort) is not out of the question as an airline pilot (retired). See 747.terryliittschwager.com. One of these days I may even get it done (if I live long enough).

@Terry: I've seen the site, it's linked from your profile page! I just might one day taxi a small plane, that is in my daughter's hands now. I've promised to her flying lessons since she's four. She's seven now and more eager than ever. Sometimes I think that we have children in order to live our dreams through enabling them.

It might have been fun and hardly noticeable 30 feet up on the end of a nice springy nose gear oleo but the SLF down the back hate it.  Krebump, kerbump, kerbump, kerbump...... :)

Was just going through this post again and had an interesting question. if you're really bored  Do you really have time before take off to get bored ? :p

@Iceman Good point! I tend to use that expression when, as I think about it now, there are better ways to say what I mean. What I'm meaning to say is that here's something you can do if you have the time and attention span to do so, but I'm not so presumptuous as to say you should or need to do it. I was never bored, but when you're on a long taxi, especially when you're in a lineup, the workload is not high. The taxi checklist is done, you won't be calling for the takeoff checklist until you reach the end of the runway, and a little refining of your taxi skills is a worthwhile activity.

Airbus documentation claims the pilot can see 20° below horizon directly ahead (provided the seat is adjusted as recommended). Given the window of A320 is about 4m above ground, the closest point is about 11m ahead of them. And the nose wheel is more than 2m behind where the pilots sit. In larger aircraft it's of course proportionally further.
So it's not that bad, but there is some problem. Out on the large taxiways seeing how the yellow lines extend to distance. It's really similar to a car. The closest point pilots see is further away, but the taxiways are proportionately larger.
Near the terminal or assigned apron spot, external guidance is usually provided. The simplest method is by ramp marshal:

Large airports usually have visual docking guidance system that serves the same purpose:

Several different systems exist.
Last, some new aircraft have cameras that can provide video to cockpit. A380 has a camera behind the nose wheel for more precise taxiing and camera on tail that provides better perspective for judging clearance to obstacles as while wingtips are visible from cockpit in most aircraft, that angle makes it difficult to judge the clearance.

777-300ER (and I think maybe also -200LR) has a camera on the belly as well.

The question (& other answers) assume an airliner, but since that wasn't specified, I'll chime in for some tail draggers:
For some tail wheel aircraft, the short answer is they don't.  If you watch a Pitts Special, a P-51 Mustang, a Gee-Bee, or other large engine wheel in the back airplane, you'll see them make a series of "S-turns" as they taxi.  The pilot sits too low to see forward past the engine when all 3 wheels are on the ground.
The solution is to "zig zag" down the taxiway.  That way you can look out one side then the other & have some idea where you are.

I was watching videos about P47s in WWII and they solved that problem by having a soldier lie on the wing while the pilot taxied out. There were dozens of planes that took off together and the soldiers efficiently guided the pilots to the runway.

Even more important than taxiing, because of lack of forward visibility, a 3-point landing in a taildragger is done using mostly peripheral vision, keeping the plane centered between the runway side edge markings.

Lets turn this question around.  How do you stay in your lane when driving (not everyone can do this well)?  Unless you are looking directly to your near side, you can't see the line on that side and you certainly cannot see the lines on the far side.  So how do you stay centered in the lane?  You use context cues and spatial awareness, extending the lines that you see the places they go that you cannot see.  You know your vehicle and roughly where the lines you can see need to be visually placed to ensure you are in the lane.
Airplanes work the same way.  We see the taxiway markings ahead of us and we can infer where they are under us.  We know our airplanes and where these markings should be relative to cues we have in the airplane to put the plane on the centerline.  If there is one thing pilots tend to be good at, it is spatial awareness and keeping the airplane on the centerline is on application of that skill.

That absolutely makes sense, thanks!

I will say, though, that I found it a lot easier to keep a car in the lane when I was first learning to drive than to keep an airplane on the center line when I was first learning to fly. Keeping it on the taxiway wasn't hard, but developing enough spatial awareness to keep it consistently on the center line took a little more getting used to than it did in a car. Part of that has to do with the need to keep your focus further in front of you than you need to do in a car. When I first started, my tendency was to keep myself (rather than the center of the airplane) on the center line.

Spatial awareness is fine when there is enough margin and/or you know the plane or vehicle very well. When you take truck or coach driving lessons, you learn to use the rear-view mirrors to precisely align (and verify) your vehicle on the centre of the lane. You need to calibrate yourself before the spatial awareness works.

The B777-300 has a Ground Maneuvering Camera System (GMCS) which can be displayed in the flight deck.
It is mainly used to aid in turns (due to the length of the fuselage), but also works well for keeping it on the center line in straight ahead taxing.
Ground Camera Video

In my experience taxing 727, 737, 757, 767, DC-9, MD80, L-1011 - if you kept the centerline between your legs you'd be ok. Sounds odd now that I type this and read it, but it worked.

You seem to be saying that you don't really need to travel down the exact centreline, which is fine, but doesn't actually answer the question of how pilots do seem to do a pretty good job of following the exact centreline.

@DavidRicherby he's actually spot on how we follow the exact centerline.  The trick is we don't try to be exact but it ends up looking that way; we use rules of thumb like where to put the centerline relative to the panel or your legs.  These rules of thumb tend to be good, which is why it appears we have some other special trick to exactly follow the line.

@casey In that case, the answer should be edited to clarify this. (Well, actually, your own answer makes it much clearer.)

To me it is just one of those things you get used to doing.
When I first started flying I would concentrate very hard while taxing and sometime miss the forest for the trees (what there was a plane at that intersection!!). But, after a while it becomes second nature.
A bigger issue was using your feet instead of your hands to turn left or right.  Also, learning to use differential thrust to bring a side around (turn left, increase right engine power) takes a bit of time.  And correct aileron position in gusty winds can also be fun.
Since my experience is all in piston single and twins, and a few light turboprops, I wonder if the big boys have the same issues.

And correct aileron position in gusty winds can also be fun: Shouldn't ailerons not be used for turning on ground? they don't seem to provide any assistance do they?

@Hanky웃Panky: Ailerons are not used for turning on ground, but during take-off run in cross-wind they need to be suitably positioned to prevent wingtip strike as the wind will tend to roll you over as you rotate.

Well, airliners (that @Hanky웃Panky apparently meant) have nose wheel steering connected to separate tiller, so you taxi with outboard (i.e. left on captains side, right on F/O's side if installed) hand and don't need to bother with differential thrust nor differential braking there (most of the time; pilots still know it for the case nosewheel steering fails).

With light planes you need to use ailerons and elevators when taxing in gusty winds.  But not to assist in turn.  Rather you want to have ailerons and elevator position such that they appropriately counteract the lift generated by a gust.  This site, http://freepilotinfo.blogspot.com/2010/01/remembering-correct-x-wind-taxi.html, has the classic diagram for appropriate aileron and elevator position while taxing.  The trick is remember to change whenever you make a turn or the wind shifts.


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What is the manoeuvre point?

a centre of gravity position at which stick force per g becomes zero

What does this mean? If the centre of gravity happened to be at the maneouvre point, would there be stick be very light in the hands? If this was the case would an aircraft be very responsive to stick inputs in pitch changes?

You should include the source of that statement.

@John K. Why? Seriously? It is a definition. It would be akin to giving a source to the definition of ‘centre of gravity.’

That's fine but you normally provide a link to where it was copy and pasted from, or a reference to the source just as a formality.  Like "Oxford Dictionary defines so and so as whatever....".  Someone may want to look up the material themselves for whatever reason.


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How does the power distribution change for a turbofan engine from being stationary to going at full speed?
I have found out that a turbofan engine of say an F-16 is producing zero power when stationary since $Power = Thrust × Velocity$. But assuming that the engine is producing the same thrust, it appears that regardless of whether the F-16 is stationary or moving, the engine is still consuming the same amount of fuel and therefore there is power output that is constant in both cases. I also found that the power I tried to calculate from the equation above is the propulsive power, so there must be something else that is consuming the rest of the power.
To make this simple, I will use some numbers. Let's assume a turbofan engine is capable of producing 100 kN of maximum thrust with no afterburner while consuming  4 kg of fuel per second. For the sake of simplicity I'm gonna use a figure of 30% thermal to mechanical efficiency for the engine that is constant through all of the following cases and which means that  there is 30% $×$ 4 kg $×$ 42  MJ/kg = $50 MW$ of mechanical power available at all times. With 43 MJ/kg being the heat value of the jet fuel.
Case 1: The engine is stationary and producing max thrust and therefore the propulsive power is zero so I assume that the 50 MW available is completely used in heating and accelerating the exhaust.
Case 2: The aircraft is accelerating and is now at half its max speed at sea level which is say 200 $m/s$. The propulsive power is now = 100,000 kN × 200 = $20$ $MW$ and so I'm also assuming that the other 30 MW remaining of the 50 MW available are still being consumed in accelerating and heating the exhaust.
Case 3: The aircraft reached its max speed at 400 $m/s$ at sea level. The propulsive power is now = 100,000 kN × 400 = $40$ $MW$ and as before, 10 MW are consumed by the exhaust.
So, if I understand this correctly, the power distribution goes from 0% propulsive power and 100% exhaust power when the engine is stionary to 40% propulsive power and 60% exhaust power when the aircraft is at half full speed and continues to shift the power available to propulsive power at higher speed until it is 80% propulsive power and 20% exhaust power. My question now is: In simple terms and with acceptable approximations, does this shift that redistributes the power from exhaust to the propulsive power as the speed increases really happen?

Increasing speed increases the aircraft "power" by definition. Constant thrust (from the jet engine) will make the aircraft accelerate until drag brings it to a steady state speed.  This is why we can differentiate engine output as thrust/time compared with fuel burn/time and "power" = Thrust x Velocity as its energy state.

@RobertDiGiovanni, jet engines don't produce anything close to constant thrust by long shot. For pure turbojets and low bypass turbofans the increase in efficiency due to pressure recovery is however significant enough that the peak thrust is not at standstill. High-bypass turbofans behave more like props.

@Jan Hudec never said they always did, see sentence 3 paragraph 2 of my answer.  The point is:  that even with perfectly constant thrust, the Power, by definition, increases as velocity.  Without going back to James Watt, or treating jet propulsion differently (why should we?), thrust will always be energy released from fuel burn (with an efficiency factor)- friction and drag from running the engine.  This is different from the energy state A x V of the object being propelled and should not be confused. The Force from drag/friction resulting from movement brings it to steady state velocity.

Your definitions are correct and your derivations correctly use the law of conservation of energy, so yes, the power distribution change does happen.
The more important question is what does this actually mean. Energy is a rather curious quantity in that while it is conserved in each reference frame (including non-inertial as long as appropriate potential is included), the values of kinetic and potential energy will be different in each!
You can look at all the cases in the reference frame of the aircraft and then the propulsive power will always be zero, because the velocity is—by definition of the reference frame—zero. But the very same situation viewed from the reference frame of the Earth it will be as you describe, and from the reference frame of the air mass it will be similar, but the values will differ slightly as the reference frame move relative to each other at the speed of the wind.
Note that enthalpy does not vary in coordinate transformation, so the sum of the power towards moving the aircraft and towards accelerating the air stream is will be the same in all the reference frames. Most easily calculated in the reference frame of the aircraft from difference in kinetic energy between the intake and the exhaust. This is the output power of the engine itself, and comparing this with the heating value of the fuel will give you the thermodynamic efficiency. Thermodynamic efficiency of turbine engines increases with flight speed due to pressure recovery, i.e. the ram air pressure increasing the effective compression ratio.
Also note that this variance in propulsive power is there for all kinds of propulsion systems, including propellers and wheels of ground vehicles. The propulsive power is always zero at standstill, and how quickly it increases depends on the reactive mass they have available. The more reactive mass, the larger the propulsive efficiency (= propulsive power to output power) will be, and the faster it will increase with velocity. Propellers have some more reaction mass available due to larger diameter, and wheels have whole Earth available, but their thrust is still limited by the surface friction and the maximum torque of the engine (and the friction of the clutch for engines that don't produce torque at zero RPM).

In your calculations, you multiply the jet engine thrust with the airspeed in order to get to propulsive power. Jet engines produce relatively constant thrust at all airspeeds - it goes against our intuition to define propulsive power as zero with the engine at full thrust on the runway before take-off, and I assume that that is the basis of your question.
In order to get from thrust in [N] to power in [W] we indeed can multiply a force with a velocity, but it makes much more sense to take the velocity of the outflowing exhaust gas stream (relative to the aircraft exhaust) for this calculation. You'll find a much more consistent efficiency: the internal fuel energy is now converted into heat + kinetic energy of the exhaust gas.

Actually, jet engines don't produce anything close to constant thrust by long shot. For turbojets, the pressure recovery is very significant, which means their thrust first declines a bit from standstill to about 0.3M, then increases again, peaks somewhere between maybe M1.5 and M2.5 and only then falls off as the flight speed approaches the exhaust speed. Turbofans behave more like props and their thrust declines faster due to the much lower exhaust speed.

This is where physics can be really fun.  And there certainly are many possible interpretations here.  The definition of Power = Thrust x Velocity = Mass x Acceleration x Velocity may more correctly apply to the energy of the F-16 as an impactor crashing into something rather then it's power output from burning fuel.
Engine power output may be more aptly described as Thrust + Compressor Drag.
This is a much simpler addition of forces.  Notice once the plane is moving and achieves steady state velocity, efficiency may increase due to ram air unloading the compressor, but the thrust required to maintain that speed will still be equal to the drag of the aircraft.
Notice, as we move to fans and turboprops, fan or prop "drag" becomes the dominant propulsive source. (Prop airfoil "lift" = "drag").

Power and force are different dimensions, so you can't make the definition you are trying to in the second paragraph. The definition in the question is the correct one anyway (mind the “propulsive” qualification).

In reality an engine produces thrust.  Rocket, jet, prop, you name it.  An examination of fuel burn vs thrust is efficiency.  Movement and energy state of the propelled object are covered by A x V and are expressed as power.  Because power can vary with speed at constant thrust, and drag factors are involved with movement, these are two different subjects.  That is the point.

Keep in mind that efficiency is a well defined term in physics that always means ratio of powers. Thrust specific fuel consumption may be a better comparison of jet engines, especially turbojets, than efficiency, and is usually used for the purpose, but it shouldn't be called efficiency. And a power output still has to be power, not force (power output of jet engine is the change of energy of the working fluid, power output of the core is the change in energy of the working fluid in the core plus torque times rpm on the fan).

I will go with thrust specific fuel consumption for all engines, not just jets.  Reading Koyovis as well, thanks.  See Kampf.


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Does the P&W F100 turbofan engine of the F-16 really produce this much power?
I've been trying to calculate the power a P&W F100 engine on a F-16 is producing using some of the available data on thrust and velocity and I ended up with a number that I want to confirm. Here's how I made the calculations.
First of all, I don't want my calculations to involve any complications due to the altitude and its effect on the engine thrust, so I just chose to make my calculations at sea level. Wikipedia states that the maximum speed of an F-16 at sea level is 1,470 $km/h $ which is ~$408$ $m/s$.
Secondly, I found out that jet engines in general produce different thrust at different speeds. I did some research and found that turbojets produce somewhat  constant thrust at subsonic speeds and gain extra thrust at supersonic speeds, while turbofans suffer degrading thrust with speed with a degree depending on the bypass ratio of the turbofan. I found that high bypass ratio turbofans suffer much more thrust degradation at high speeds compared to low bypass ratio turbofans. which means that low BPR turbofans fall somewhere in between the turbojet and high BPR turbofans according to their BPR.
I found that P&W F100 Wiki gives a bypass ratio of $0.36:1$, then looking at this thrust vs speed graph, which confirms what I concluded above, I could determine that the F100 engine with 0.36 BPR can have no more than 10% thrust degradation at mach 1.2. I also think that using the afterburner shifts the curve further up since more air is now entering the core than before without increasing the fan flow and therefore the engine behaves even more like a turbojet than a turbofan. Overall, I think it would be safe to say that the afterburner thrust at Mach 1.2 at sea level will not degrade more than 10% and so the F100 engine thrust will become 0.9×127 kN = $114.3$ $kN$
Now the power is simply $P = F . v$ = $(114.3 × 10^3) × 408$ = $46.6 × 10^6$ $W$ or about $62,000$ $hp$.
So my question now is: Are my calculations acceptable to a certain degree or are they completely off and I didn't take into account another significant factor?
EDIT: I have another question. I'm assuming here that the engine is producing full thrust at the sea level to counter the air drag and achieve the quoted speed. But is this a correct assumption? In other words, could an F-16 fly at its maximum speed at sea level while not using the maximum thrust?

In answer to your edit, it is possible for an aircraft to exceed is maximum speed (e.g. a limit speed - Vmo/Mmo) while not using max thrust. In an aircraft designed for supersonic flight like the F-16, the maximum theoretical speed is probably much closer to its design limit speed than it would be in an airliner, but there will be a difference

Also, the first question is more of a physics question than an aviation one, while the edited one is more aviation-related

https://en.wikipedia.org/wiki/Pratt_%26_Whitney_F100 gives a maximum afterburning thrust for the F100-PW-220 of 105.7 kN, so you were pretty close there.  Power is not listed on wikipedia, but you can get there.  It lists "Specific fuel consumption: Military thrust: (0.73 lb/(lbf·h))", so all you need to know is the lower heating value of jet fuel, and you can calculate the power. Off the top of my head, 46 MW seems a little high for that size engine, but it's certainly on that order of magnitude.

I think the data on the F-16 Wikipedia is about the F100-PW-229. The engine WP says it is used for late model F-16s: https://en.wikipedia.org/wiki/Pratt_%26_Whitney_F100#F100-PW-229. Or it could be the F110-GE-129: https://en.wikipedia.org/wiki/General_Electric_F110#F-16
In any case, I think 127 kN is within the possible range for an F-16 and using it at the mentioned speeds wouldn't be incorrect.

If I understood you correctly, I found that the lower heating value of jet fuel to be 43.1 MJ/kg. For the F100-PW-229, the afterburner SFC is 1.94 lb/(lbf·h) which for 127 kN (29,000 pounds-force) would be 6.56 kg/second. This would generate power of 282 MW which is much more than the value I calculated and which shows that not all of the fuel chemical energy is utilized as mechanical work. Please correct me if I'm wrong or if I misunderstood something you said.

@AbanobEbrahim: That points to a thermal efficiency near 17%, which is entirely believable.


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What is the best aircraft by useful load fraction?
I’m looking for the most capable in terms of payload relative to own weight heavier than air vehicle of any type, size or era. Untested designs are of interest too.
Essentially, what is the most efficient way to fly?
I suppose that ultimate vehicle is probably made of top-end composite materials, uses high lift devices and is electric as batteries doesn’t count to the empty weight.
I have done a fair research on the topic from unmanned rc models to Stratolaunch, and so far the best discovery is https://www.nestofdragons.net/weird-airplanes/special-facet-opal/ with 1.73 ratio.
This particular aircraft is a very old design and quoted numbers may be not realistic, so I wounder what is the modern record keeper?

Take a letter, fold it into a paper airplane and throw it. Payload fraction = 1.

Batteries don't count towards the empty weight?!  Because unless you have the option to remove them they are part of the aircraft and reduce your payload.

Off the top of my head Rutan's Globalflyer: empty weight 1'700kg, take off weight 10'000kg i.e. 0.17 empty-weight fraction. The aircraft you linked had an empty-weight fraction of 0.4 a quite extreme value for homebuilt composite airplane... its end might suggest that it was indeed structurally too extreme

fuel apparently also counts as useful load, hence no need for electric. There's that thing with counting fuel / batteries as useful load, it's really a bit pointless. What you want really is

Judging from your comments, you want best payload weight per unit of drag (which equates to "best energy per payload per distance"). Please edit your question accordingly.

The best "payload mass per unit of drag" will get you something like a space shuttle since drag is virtually zero. In a practical sense it's not the most efficient way to airlift some payload from one place in the world to another.

I guess the lightest aircraft w.r.t. payload will be something like a paraglider .. as an example:
ozone XXLite 19: Glider weight 1.37 kg for payload (pilot) 115kg (https://www.paraglide.co.uk/store/ozone-xxlite-lightweight-paraglider-p-117), so that's approximately 1.2%.
I assume that even with a paramotor the ratio $m_{payload} / m_{TOW}$ still holds as "quite good".

well, technically this is correct answer, but so would be a tethered kite with even higher useful payload fraction.

There are also very capable wing-shaped airships filled with helium which are net havier than air utilizing aerodynamic lift to fly, but I guess the word "rigid" is missing in the title to meet original expectations

Ok - first, commenting on the kite: i don't know why a tethered kite would have a higher useful load?

Concerning the rigid aircraft .. a counter question: is there a specific reason it needs to be rigid?

The question is about most efficient way to fly, and all those strings on a paraglider and pilot's harness makes a lot of drag.
I think I'm looking for best energy per distance per payload.

Well .. here we go. Efficiency alone for an aircraft is apparently not defined clearly enough. Your question was about useful load / mTOW. If you want aerodynamic efficiency, go for a high glide ratio. A useful definition of powered flight efficiency might be something like payload*range / fuel_consumed, but you might want to throw some cruise speed requirements in the mix. The point I want to make is that the optimum is usually strongly dependent on mission requirements.


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Ventral masts- what are they?
I have kids working on a model of a Boulton Paul Defiant, a WWII-era fighter plane. When we found images of this plane, it shows two rods sticking vertically down from the belly of the plane. Some of the images show them to be quite long-- the plane would have a hard time landing if they remained that length! After doing some searching, it seems like these are called "ventral masts". What are these for? Do they retract for landings? Thank you!

Can you add an picture to illustrate your question?

+1 for getting kids into modeling! Way to go!!

Normally, those masts are located on the upper fuselage and sometimes on the wings. They are used to string antennas between them and other airplane parts, or are antennas themselves.
In case of the Boulton-Paul Defiant, those antenna masts could not be located in the upper hemisphere in order to keep the field of fire of its gun turret free. Together with the Blackburn Roc and the Hawker Hotspur, those airplanes had a unique layout for a fighter with a single pilot and a gun turret placed behind the cockpit. How well this idea turned out in practice can be read in this answer.
In case of the Defiant, those antenna masts were made retractable to allow take-off and landing. The antennas were needed for communication and navigation.

The different antennas on a Grumman TBF Avenger (picture source).

During the era of the 2nd World War, some aircraft, like the Defiant, had radio masts that were used for communication and navigation. They could be extended while in flight to increase receptive range and retracted before approach to clear them out of the way for landing.
Aircraft Anatomy of World War II: Technical Drawings of Key Aircraft by Eden and Moeng is a good resource to see how elements like this were incorporated into the myriad different aircraft of the time period.
Hope this helps. Happy modeling.